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I am rebuilding a 49 series v twin 27hp that broke a rod. Can the head bolts and rod bolts be reused? Or are they torque to yield?
Can regular grade 8 bolts be used instead?

I’ve never had a problem reusing head bolts. Just make sure they are clean and dry before installing. New rod should come with new bolts.

I have reused bolts for 65 years, no problem except I did buy new grade 8 bolts from local farm supply store when installing new sump gasket in V Twin B&S because of greatly increased torque spec. due to chronic sump gasket leaks with 40 and 44 series engines.
Walt Conner

Rivets said:

I’ve never had a problem reusing head bolts. Just make sure they are clean and dry before installing. New rod should come with new bolts.

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The only time I haven't reused head bolts unless they came with a new head was on the old K series Kohlers. Something about the aluminum to cast transition would cause the head gasket to blow if you reused the bolts.

Rivets said:

I’ve never had a problem reusing head bolts. Just make sure they are clean and dry before installing. New rod should come with new bolts.

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It’s a used rod I am using

Tinkerer200 said:

I have reused bolts for 65 years, no problem except I did buy new grade 8 bolts from local farm supply store when installing new sump gasket in V Twin B&S because of greatly increased torque spec. due to chronic sump gasket leaks with 40 and 44 series engines.
Walt Conner

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This is the problem this engine had. The factory sump bolts are fully threaded. Did you use partially threaded bolts from the farm store?

Tinkerer200 said:

I have reused bolts for 65 years, no problem except I did buy new grade 8 bolts from local farm supply store when installing new sump gasket in V Twin B&S because of greatly increased torque spec. due to chronic sump gasket leaks with 40 and 44 series engines.
Walt Conner

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Did the torque spec change? What was it?

Rivets said:

I’ve never had a problem reusing head bolts. Just make sure they are clean and dry before installing. New rod should come with new bolts.

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I thought some type of lock tight was supposed to be used?

It was 4 years or more ago that I bought grade 8s from Rural King, I don't remember for sure but I do not believe they were fully threaded but as far as was needed.

I am 89 and have not worked on engines for a few years but I think original torque was 120 inch lbs. and went to 200 inch lbs when they introduced their "fix" kit which was $35 - $40 last I knew. Thoroughly discussed with that outrageous price, I made my own gasket, bought new grade 8s and torqued to new setting, no problems so far.
Walt Conner

No Loctite, and most head bolts are grade five, partially threaded. Just make sure they are clean, dry and in good condition. Also make sure the holes are clean and dry.

Rivets said:

No Loctite, and most head bolts are grade five, partially threaded. Just make sure they are clean, dry and in good condition. Also make sure the holes are clean and dry.

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There were six marks on the bolts so I assumed grade 8….

You are correct.

unless the bolts are corroded, look fatigued or crystalized, or they feel like they are stretching when they are tightened I would use the old ones. As to rod bolts it is a crap shoot. I have seen new ones stretch below the recommended torque, and I have seen the old ones fail. Anyone remember the Jesus bolts on the L1011's where the engines fell off? Not all bolts are created equally.

Before you stretch the bolts on an alloy mower engine you will cause the hole to collapse in on itself, which is the way you know if they have been overtightened in the past .
As for thread length put the bolt through the head / rod .
If the thread is better than 1 full turn inside the head / rod then there is sufficient thread to torque the head / rod .
A mower head is roughly done up till the parts have touched & the bolt stops turning easily ( finger tight if you like ) then 1/3 a turn in 2 x 1/6 turn increments